1. Field of the Invention
The present invention relates to a cutting tool made of a surface-coated cubic boron nitride-based ultra-high-pressure sintered material (hereinafter referred to as a surface-coated cBN-based sintered tool) comprising a cutting tool body made of a cubic boron nitride-based ultra-high-pressure sintered material and a hard coating layer provided on the surface of the cutting tool body, which comprises a hard coating layer having excellent high-temperature hardness, high-temperature strength, heat resistance and edge notching resistance, and also can exhibit excellent edge notching resistance and maintain excellent performance (e.g. providing an excellent surface finish of workpieces) over an extended period of time even in a high speed cutting operation of a hard steel, such as alloy steel or hardened bearing steel.
Priority is claimed on Japanese Patent Application No. 2006-147300 filed May 26, 2006, the contents of which are incorporated herein by reference.
2. Background Art
A surface-coated cBN-based sintered tool in general includes an insert that is removably attached at the tip of a cutting tool for turning of workpieces made of various types of steel or cast iron, and an insert type end mill that is removably attached with an indexable insert and is used in chamfering, grooving and shouldering of workpieces in a manner similar to that of a solid end mill.
One known constitution of surface-coated cBN-based sintered tools includes a tool body made of various cubic boron nitride-based ultra-high-pressure sintered materials (hereinafter referred to as a cBN-based sintered material) and a surface-coating layer such as a titanium nitride (TiN) layer or a titanium aluminum silicon nitride ([Ti, Al, Si]N) layer formed by vapor deposition on the surface of the tool body. It is also known that these surface-coated cBN-based sintered tools are used for cutting workpieces made of various types of steel or cast irons (Japanese Patent No. 3460288).
It is also known that the above-described surface-coated cBN-based sintered tool can be manufactured by coating the surface of the tool body S with a layer of a suitable composition, for example, a TiN layer or [Ti, Al,Si]N layer using an arc ion plating apparatus, as a variation of a physical vapor deposition apparatus schematically exemplified by FIGS. 1A and 1B. For example, the surface coating layer may be formed by setting the tool body in the arc ion plating apparatus; generating arc discharge by supplying a current of 90 A between anodes 61,62 and cathodes (evaporation source) 31, 32 consisting of metallic Ti or a Ti—Al—Si alloy of a predetermined composition, where the ambient temperature is maintained at, for example, 500° C. by a heater, while nitrogen gas is introduced as a reaction gas into the apparatus so as to achieve a reaction atmosphere with a pressure of 2 Pa, and a bias voltage of, for example, −100 V is applied to the tool body (Japanese Patent, No. 2793773).
There have recently been remarkable developments in both the design and installation Factory Automatization (FA) of cutting apparatuses. On the other hand, there are still strong demands for labor saving, energy saving and cost reduction in cutting operations. Therefore, in addition to a cutting operation under ordinary conditions, there is an increasing demand for a cutting operation under high-speed cutting conditions. The conventional surface-coated cBN-based sintered tool performs satisfactorily in machining of steel and cast iron under ordinary cutting conditions. However, when it is used in a high-speed continuous cutting operation or a high-speed interrupted cutting operation of a hard steel (steel having high hardness), such as alloy steel or hardened (quenched) bearing steel which has a Rockwell hardness (C scale) as high as 50 or more, damage (hereinafter referred to as edge notching) occurs at the boundary portion of the cutting edge by high degree of heat generated at a cutting edge or a drastically large mechanical load is imposed intermittently and impulsively at the cutting edge. As a result, it becomes difficult to maintain the surface finish of workpieces when the tool is used over an extended period of time. Thus the cutting tool reaches the end of its tool life in a relatively short period of time.